1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus incorporating the zoom lens, which can be applied to digital cameras, video cameras, silver-salt photograph cameras, and so forth.
2. Description of the Related Art
In recent years, with image pickup elements employed for an image pickup apparatus such as a digital camera, high pixelation has advanced. In response to this, high resolution has been demanded for imaging lenses (imaging optical systems) employed for image pickup apparatuses including high pixel image pickup elements. In order to realize a high resolution imaging lens, it is useful for various aberrations relating to image capabilities at a single color (single wavelength), such as spherical aberration and coma aberration, to be corrected well. In addition to this, it is useful for an image obtained when employing white illumination light to have well-corrected chromatic aberration so as not to exhibit chromatic bleeding.
On the other hand, in order to enlarge a photographing area, there is demand for imaging lenses to serve as a zoom lens which can have a high zoom ratio. Generally, further lengthening of the focal length at a zoom position at the telephoto end to obtain a high zoom ratio causes an increase in the fluctuation in chromatic aberrations due to zooming and an increase in chromatic aberration of magnification and axial chromatic aberration to occur at a zoom position at the telephoto side.
Accordingly, in order to obtain imaging capabilities of high image quality, it has been important to appropriately perform not only primary spectrum correction but also secondary spectrum correction for correction of chromatic aberration. Note however, attempting to obtain a high zoom ratio causes an increase in the axial chromatic aberration and chromatic aberration of magnification of the secondary spectrum particularly at the telephoto side, which has been difficult to be corrected well.
Heretofore, disposing a lens configured of glass with anomalous partial dispersion within a lens unit at the object side of lens units constituting a zoom lens has been known to correct the chromatic aberration of the secondary spectrum at the telephoto end.
Also, a zoom lens has been known wherein chromatic aberration is corrected by employing a diffraction optical part (diffraction optical face). Providing a diffraction optical part within a lens unit at the object side enables occurrence of chromatic aberration to be reduced at the telephoto side according to the anomalous dispersion effects thereof.
Note however, in a case wherein a diffraction optical part is provided within a lens unit at the object side to correct chromatic aberration at the telephoto side, this diffraction optical part has a few correction effects regarding chromatic aberration at the wide-angle end. Conversely, when attempting to extremely improve chromatic aberration at the telephoto side by increasing the refractive power of the diffraction optical part, the chromatic aberration at the wide-angle end increases.
On the other hand, providing multiple diffraction optical parts within an optical system facilitates chromatic aberration to be corrected at the wide-angle and telephoto end. Of these, a zoom lens has been known wherein multiple diffraction optical parts are disposed before and after an aperture within an optical system (Japanese Patent Laid-Open No. 2004-117826, Japanese Patent Laid-Open No. 2004-117827, and U.S. Pat. No. 6,154,323). Note however, when providing multiple diffraction optical parts within a zoom lens, setting the refractive power thereof and the positions within the optical system thereof improperly makes it difficult to greatly improve chromatic aberration.
With Japanese Patent Laid-Open No. 2004-117826, Japanese Patent Laid-Open No. 2004-117827, and U.S. Pat. No. 6,154,323, multiple diffraction optical parts are provided, but the power thereof is not necessarily arranged to be the optimal relation for correcting chromatic aberration at the telephoto end and wide-angle end. Also, with the positions of the diffraction optical parts within the optical system as well, it cannot be said that priority is given to the correction of chromatic aberration at the telephoto end and wide-angle end.
In order to increase a zoom ratio, and also correct chromatic aberration over the entire zoom range well to obtain high optical performance, it is important to set the positions within the optical system where the diffraction optical parts are provided, the power thereof, and so forth appropriately.
Now, a zoom lens having a four-unit configuration with a high zoom ratio has been known, which is configured of lens units having positive, negative, positive, and positive refractive powers in order from the object side to the image side, and performs zooming by moving each of the lens units (U.S. Pat. No. 7,253,965). Also, a zoom lens having a five-unit configuration with a high zoom ratio has been known, which is configured of lens units having positive, negative, positive, and negative refractive powers in order from the object side to the image side, and performs zooming by moving each of the lens units (U.S. Pat. Nos. 6,788,464 and 7,133,213).